Method of straining fibrous suspensions



Oct. 15, 1946a 5 AHLFQRS 2,409,524

METHOD OF STRAINING FIBROUS SUSPENSIONS Filed Jan. 30. 1942 Patented Oct. 15, 1946 METHOD OF STRAINING FIBROUS SUSPENSIONS Sten Eskil Einarsson Ahlfors, Hedemora, Sweden Application January 30, 1942, Serial No. 428,930 In Sweden July 22, 1940 4 Claims.

This invention relates to straining of fibrous suspensions, particularly paper pulp.

One of its objects is to provide a straining method resulting in a thickening of the pulp strained as compared with the pulp to be strained.

Other objects are to increase the output of the strainer and decrease the weight thereof.

The invention consists in the novel method hereinafter fully described and particularly pointed out in the claims hereunto appended.

The invention is illustrated by way of example on the annexed drawing in which:

Fig. 1 shows a longitudinal section along line I--I in Fig. 2 of an embodiment of my improved strainer;

Fig. 2 shows a transverse section along line IIII in Fig. l of the strainer illustrated in Fig. 1.

Fig. 3 shows an enlarged fragmentary plan view of the bottom of the strainer vat and one of the sprinkle tubes in Fig. 1.

On the drawing i represents the supporting frame of the strainer, 2 the stationary pulp vat and 3 the vibratory strainer vat. The latter is resiliently supported by means of springs d depending from the frame I. Associated with the strainer vat 3 is an actuating mechanism therefor comprising a rotary shaft 5 journalled in bearings 6 supported by the strainer vat 3 by means of longitudinal and transverse bars I and 8 respectively. Carried by the shaft 5 intermediate its ends is a pulley 9 adapted to be rotated by means of a belt I from a source of power not illustrated. At its ends the shaft supports unbalanced or eccentrically arranged masses H serving to vibrate the strainer vat 3 in rotating the shaft 5. The bottom l2 of the strainer vat 3 forms the strainer proper and is provided with slots i3 extending in the longitudinal direction of the strainer from the lower surface of the strainer bottom l2 and communicating with recesses M at the top surface of the strainer bottom said recesses successively increasing in width from the upper end of the slots it to the top surface of the strainer bottom I2.

The pulp vat 2 is provided with an outlet 15 for the material rejected by the strainer, and at Hi there is indicated a stationary suction tube for discharging the strained pulp.

Associated with the strainer vat 3 are spouting or sprinkling members consisting of a plurality of tubes ll spaced at some distance from the top surface of the bottom E2 of the strainer and extending thereacross. Said tubes are adapted to be supplied with liquid under pressure either continuously or intermittently by any suitable means not illustrated on the drawing and are provided with a series of discharge apertures or nozzles 98 pointing towards the recesses 14 and extending in a direction sloping towards the end of the strainer bottom [2 where the discharge tube It is situated.

The embodiment of my improved strainer above described operates as follows:

The pulp to be strained is supplied to the pulp vat 2 at any suitable point, such as at I9 in Fig. 1 and the supply of the incoming pulp is so controlled as to maintain a suitable level of liquid in the pulp vat 2 as indicated at a in Figs. 1 and 2. The pulp thus introduced tends to move upwards through the slotted strainer bottom l2 which is caused to vibrate rapidly by means of the actuating mechanism above described. The matter passing through the slots 13 is continuously discharged at the top face of the strainer bottom I2 by the suction tube [6. On the other hand the matter rejected by the strainer, such as knots or ogher coarse matter is continuously discharged at In practising the above method of straining I have found that by suitable choice of the operation conditions, particularly of the characteristics of the vibratory motion of the strainer Vat, the pulp strained will be thickened as compared with the entering pulp to be strained whereas the pulp in the vat 2 becomes thinner during the course of the straining. This action is contrary to what happens in all straining methods hitherto known in which the entering pulp becomes thicker as the straining proceeds which as a rule necessitates the addition of larger or smaller quantities of diluting water at the input side of the strainer to prevent cloggingof the strainer at that side. Such addition of water for diluting the pulp in the pulp vat is entirely avoided by my present method of straining. Besides, if desired thicker pulps can be strained according to my method without incurring the risk of clogging of the strainer at the input side. This means a considerable increase of the output of the strainer per unit of surface of the strainer area.

In practising my improved method I have found that the velocity of the vibratory motion of the strainer must be relatively high to obtain satisfactory straining. Thus, in practice I have obtained excellent results of straining, for instance, by using a frequency of vibration of about 1200-1400 vibrations per minute and an amplitude of vibration of about 2-5 millimetres. Greater or smaller departures from these values are, however, permissible in practice depending on the characteristics of the entering pulp and the desired pureness of the strained pulp. As a rule I have, however, found that the frequency of vibration should not be diminished below about 900 vibrations per minute as the output of the strainer then materially decreases, and on the other hand an increase of the frequency of vibration to a value above about 1800 vibrations per minute has been found to appreciably impair the straining effect.

During the straining operation water is ejected cording to the present process is thickened, that is partly dewatered during the straining operation. In the embodiment shown merely the-recesses It need be scavenged by the jets of water projected by the sprinkle system H, 18 to remove any matter collecting .in said recesses during the process of straining. Thus, when using slots increasing in width upwards as shown on the drawing the cleaning water from the-sprinkle system needs not be pressed through the strainer slots from the top face of the strainer to the bottom face thereof which generally might cause a reduction of theoutput of the strainer. .The .water of the jets sweeping clean the recesses M is discharged through the tube l6 together with the strained pulp.

In the embodiment illustrated the jets of clean- .ing water are projected substantially in the longitudinal direction of the strainer and also in the longitudinaldirection of the recesses l4 facilitating an effective cleaning of the upper surfaceof the strainer proper and assisting in the flow of the pulp strained along the preferably inclined bottom I2 towards the discharge tube l6.

In straining paper pulp-or other fibrous suspensions according to the present method the strained pulpshould be maintained in a relatively thin layer, preferably at the thinnest possible layer on the strainer proper. This not only securesan effective cleaning of the upper face of the strainer from fibrous matter tending to clog the slots of the strainer'but also prevents an excessive dewatering of the strained pulp Which might result in an excessive deposition of fibrous matter in the recesses at the top surface of the strainer. In addition, by maintaining the thinnest possible layer of liquid in the strainer vat the entire strainer device obtains the lowest possible height. Thus, the level of liquid in the strainer vat firstly reduces the necessary height of the pulp vat containing the strainer vat and receiving the pulp to be strained as the level of this pulpwhich must-be at a certain height above the level ofthestrained pulp in the strainer vat, is lowered witha reduction of the thickness or level of the strained pulp. Secondly also the height of the strainer vat and the part thereof immersed into the fibrous suspension are reduced and the weight thereof diminished whereby the strainer vat may be vibrated more easily resulting in a reduction of the size of the unbalanced masses ordinarily used for vibrating the strainer vat, of the necessary driving power and also of the stresses to which the material of the strainer vat is subjected during the vibration.

The invention is susceptible of various changes as to the details of-the method and apparatus describedand the right is herein reserved to make such changes as properly fall within the scope of the-appended claims.

What I claim is:

1. A method of straining fibrous suspensions, particularly paper pulp, comprising supplying the pulp to be strained to the bottom surface of a strainer element having straining slots therein which have side walls which are divergent upwardly, .causing the pulp thus supplied to pass upwards through said strainer element, imparting vibratory motion of a frequency of about 1200 to 1400 vibrations per minute to said strainer element so as to cause a thickening of the :pulp when passing upwards'throughgsaid strainer element, discharging the thickened strained pulp at the top of said strainer element and the material rejected by said strainer element at the bottom thereof, the discharges being regulated so as to maintain a thin layer of strained pulp on the screen, and cleaning the slots of said strainer element by spraying liquid against the side thereof facing the pulp strained.

2. A method of straining fibrous suspensions, particularly paper pulp, comprising supplying the pulp to be strained to the bottom surface of a strainer element having straining slots therein which have side walls which are divergent upwardly, causing the pulp thus supplied to ,pass upwards through said strainer element, imparting vibratory motion of a frequency of about 1200 to 1400 vibrations per minute to said strainer element so as to cause a thickening of the pulp when passing upwards through said strainer element, discharging the thickened strained pulp at the top of said strainer element and the material rejected by said strainer element at the bottom thereof, maintaining the thinnest possible layer of strained pulp upon said strainer element, and cleaning the slots of said strainer element by spraying liquid on the side thereof facing the pulp strained.

3. A method of straining fibrous suspension, particularly paper pulp, comprising supplying the pulp to be strained to the bottom surface of a strainer element having straining slots therein which have side walls .which are divergent upwardly, causing the pulp thus supplied to pass upwards through said strainer element, imparting vibratory motion of .a frequency of about 1200 to 1400 vibrations per minute and an amplitude of about 2 to 5 millimeters to said strainer element so as to cause a thickening of the pulp when passing upwards through said strainer element, discharging the thickened strained pulp at the top of saidstrainer element and the material rejected by said strainer element at the bottom thereof, the discharges being regulated so as to maintain a thin layer of strained pulp on the screen, and cleaning the slots of said strainer element by spraying liquid on the side thereof facing the pulp strained towards the zone at which the pulp strained is discharged.

4. A method of straining fibrous suspensions, particularly paper pulp, comprising supplying the pulp to be strained to the bottom surface of a strainer element "having straining slots therein which have side walls-which are divergent upwardly, causing the pulp thus supplied to pass upwards through said strainer element, imparting vibratory motion of a frequency of about 1200 to 1400 vibrations per minute and an amplitude of about 2 to 5 millimeters to said strainer element so as to cause thickening of the pulp when passing upwards through said strainer element, discharging the-thickened strained pulp at the top of said strainer element and the material rejected by said strainer element at the bottom thereof, the discharges being regulated so as to maintain a thin layer of strained pulp on the screen, and cleaning the slots of said strainer element by spraying jets of water on the side facing the pulp strained projected in the longitudinal direction of the slots of said strainer element towards the zone at which the pulp strained is discharged.

STEN ESKIL EINARSSON AHLFORS. 

